As discussed above, previous drilling has already laid the foundation to
much of the crustal flux equation at the Izu and Mariana subduction
systems and provided a strong rationale for continuing the effort to
determine the mass balance fluxes across the subduction zones. The
missing part of the flux equation is largely the input: (1) the altered
oceanic crust seaward of the Mariana Trench--Site 801--and (2) both the
incoming sediment and basaltic sections approaching the Izu-Bonin
Trench--Site 1149. In addition, both sites are located along the same flow
line in Mesozoic Pacific oceanic crust (from ~170 to 130 Ma) and provide
an unparalleled opportunity to study the geochemical and physical nature
of old Pacific crust and its tectonic, sedimentation, and magnetic
histories.

Site 801
The primary motivation for returning to Hole 801C, seaward of the
Mariana Trench (Fig. 5), was to sample the upper oxidative zone of
alteration of this oldest in situ oceanic crust. Previous drilling during Leg
129 only penetrated 63 m into "normal" Jurassic basement. Based on
basement rocks from Hole 504B and other basement sites with sufficient
penetration, the upper oxidative zone of alteration, which contains the
lion's share of some element budgets (e.g., K, B, etc.), lies in the upper
200-300 m of the basaltic crust. The objectives of coring and logging at
this site involved

Characterizing the geochemical fluxes and geophysical aging
attending the upper oxidative alteration of the oceanic crust in Hole
801C;

Comparing igneous compositions, structure, and alteration with
other drilled sections of in situ oceanic crust (in particular Hole
504B, contrasting a young site in Pacific crust with the oldest site
in Pacific crust);

Helping to constrain general models for seafloor alteration that
depend on spreading rate and age (Hole 801C is in the world's oldest
oceanic crust that was formed at a fast-spreading ridge, so it
embodies several end-member characteristics); and

Testing models for the magnetic Jurassic Quiet Zone.

Site 801 is located in an area of very low amplitude magnetic
anomalies, the JQZ. This quiet zone has been suggested to result from (1)
oceanic crust of a single polarity with only small anomalies from field
intensity fluctuations, (2) oceanic crust with magnetic reversals so
numerous as to "cancel each other out" when measured at the sea surface,
or (3) oceanic crust with a more normal frequency of magnetic reversals
acquired when the dipole field intensity was anomalously low. Deepening
Hole 801C permitted testing of the above hypotheses, and in particular,
the third hypothesis of magnetic reversals during a period of anomalously
low field intensity as fresh, unaltered volcanic glass was obtained. Such
material can yield reliable paleointensity information (Pick and Tauxe,
1993) on the very fine, single-domain grains of titanium-free magnetite
within the volcanic glass.

Site 1149
The primary motivation for drilling at Site 1149, a site ~100 km
seaward of the Izu Trench, was to provide the first complete section of
sediment and altered oceanic crust entering this subduction zone. Previous
drilling in the Nadezhda Basin failed to penetrate resistant cherts, so
most of the sediment column is unsampled. Only 1 m of basalt has been
recovered from basement in this vast area. Core and logging data from this
site was to

1. Provide estimates of the sediment inputs and altered basalt inputs
(geochemical fluxes) into the Izu subduction zone;
2. Contrast crustal budgets for the Izu-Bonin arc with those for the
Mariana arc, to test whether along-strike differences in the
volcanics can be explained by along-strike variations in the crustal
inputs;
3. Compare basement alteration characteristics with those at Hole
801C (on 170-Ma crust along the same flow line);
4. Provide constraints on the Early Cretaceous paleomagnetic time
scale; and
5. Provide constraints on mid-Cretaceous carbonate compensation
depth (CCD) and equatorial circulation fluctuations.

In addition to serving as an important reference site for crustal inputs
to the Izu-Bonin Trench, Site 1149 can also address additional
paleomagnetic and paleoceanographic problems. Because the subduction
cycling objectives have already been discussed in some detail above (see
"Introduction"), we elaborate more in the following paragraphs on the
paleomagnetic and paleoceanographic objectives.
According to Nakanishi et al. (1988), Site 1149 is approximately on
magnetic Anomaly M12. Its basement age should be ~133 Ma and should
correspond to the Valanginian Stage of the Early Cretaceous, according to
recent time scale calibrations (Harland et al., 1990; Gradstein et al.,
1994; Channell et al., 1995). However, those age estimates are poorly
known and can be tested by drilling at Site 1149. Specifically, a
reasonably precise date on Anomaly M12 at Site 1149 could test the
proposed new time scale of Channell et al. (1995).
Based on its theoretical Cretaceous paleolatitude history, Site 1149
may have formed at ~5°S, drifted south to 10°S in its early history, and
then gradually drifted north, crossing the paleoequator as the Pacific
plate accelerated its northward motion ~85-90 Ma (Fig. 7). A site such as
Site 1149 with an Early Cretaceous basement age (~135 Ma), an equatorial
paleolatitude history during the mid-Cretaceous, and a predictable
subsidence history for the Cretaceous is ideal for testing proposed CCD
variations (Theirstein, 1979; Arthur et al., 1985). In addition, Erba (1992),
following Roth (1981), has shown that certain species of nannoplankton
can be characterized as "high fertility indices" and used as approximate
indicators of the paleoequatorial upwelling zone. Using these nannoflora,
potential fluctuations in the equatorial circulation system could be
studied at Site 1149 for the mid-Cretaceous, when it was nearly
stationary near the paleoequator (especially from 115 to 95 Ma).

Microbiology Objectives for Both Sites
The deep water (~6000 m) and proposed penetration into old oceanic
basement provided an intriguing target in the search for hidden bacterial
life forms. Leg 185 was the first ODP leg to incorporate microbiology as a
major new initiative. The microbiology objectives for Leg 185 included

1. Determining the amount of biological contamination created by the
APC, XCB (extended core barrel), and RCB coring processes;
2. Developing a sample-handling strategy for routine microbiological
sampling; and
3. Conducting culturing experiments with several media at both
atmospheric and in situ pressure.